1. Field of the Invention
The present invention relates to a membrane electrode assembly including first and second electrodes and a solid polymer electrolyte membrane interposed between the first and second electrodes. Further, the present invention relates to a fuel cell including the membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly.
2. Description of the Related Art
Generally, a solid polymer fuel cell employs a solid polymer electrolyte membrane which is a polymer ion exchange membrane. The fuel cell includes an anode and a cathode, and the electrolyte membrane interposed between the anode and the cathode. The membrane electrode assembly is sandwiched between separators (bipolar plates). Each of the anode and the cathode is made of electrode catalyst and porous carbon. In use, typically, a predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In one structure of the membrane electrode assembly (hereinafter referred to as the first structure), the outer dimension of the solid polymer electrolyte membrane and the outer dimension of the anode and the cathode are the same. In another structure of the membrane electrode assembly (hereinafter referred to as the second structure), the outer dimension of the solid polymer electrolyte membrane is larger than the outer dimension of the anode and the cathode.
In order to reduce the overall size of the fuel cell, it is desirable to reduce the thickness of the solid polymer electrolyte membrane of the membrane electrode assembly. However, in the first structure, the position of the end surface of the solid polymer electrolyte membrane matches the position of the end surfaces of the anode and the cathode. Therefore, the fuel gas supplied to the anode and the oxygen-containing gas supplied to the cathode tend to flow around the end surface the solid polymer electrolyte membrane, and the fuel gas and the oxygen-containing gas may be mixed together undesirably. Further, the short circuit between the end surfaces of the anode and the cathode occurs easily.
In the second structure, the strength of the solid polymer electrolyte membrane is low at a portion extending beyond the end surfaces of the anode and the cathode. Therefore, the solid polymer electrolyte membrane is damaged easily.
In an attempt to address the problems, Japanese Laid-Open Patent Publication No. 2003-68323 discloses a membrane electrode assembly as shown in FIG. 6. The membrane electrode assembly includes a solid polymer electrolyte membrane 1, a gas diffusion electrode layer 2 provided over one surface of the solid polymer electrolyte membrane 1, and a gas diffusion electrode layer 3 provided on the other surface of the solid polymer electrolyte membrane 1. The surface area of the gas diffusion electrode layer 3 is smaller than the surface area of the gas diffusion electrode layer 2.
The gas diffusion electrode layers 2, 3 include catalyst layers 4a, 4b and gas diffusion layers 5a, 5b. The catalyst layers 4a, 4b contact both surfaces of the solid polymer electrolyte membrane 1. The dimension of the catalyst layer 4a is different from the dimension of the catalyst layer 4b. An adhesive layer 6 is formed around the catalyst layer 4a for joining the gas diffusion electrode layer 2 and the solid polymer electrolyte membrane 1 together.
In the conventional technique, a boundary portion exists between the outer end of the catalyst layer 4a and the inner end of the adhesive layer 6. The solid polymer electrolyte membrane 1 may be damaged undesirably by the boundary portion.